The installation environment of a communication device needs to meet strict temperature and humidity requirements. However, with the development of communication technologies, the communication device may be installed in a humid environment with heavy dust and salt fog. In addition, with the upgrade of capacity of the communication device, both frame type products and box type products have higher and higher power consumption, and need fans for heat dissipation and air cooling. However, being air-cooled in an adverse environment, circuit boards of the communication device are more vulnerable to corrosion, which affects communication quality or even interrupts communication services and increases maintenance costs of an operator.
Communication device corrosion mainly results from electrochemical corrosion, and “relative humidity+dust” is a main cause for electrochemical corrosion of a circuit board. Generally, vapor can penetrate a solder mask of the circuit board, or even penetrate an organic coating material and enter a metal material surface. Scientific research shows that the critical value of relative humidity of the air that corrodes metal is: 70% RH for steel, 60% RH for copper, 76% RH for aluminum, 63% RH for iron, and 60% RH for zinc. If metal is placed in an environment where the critical value is exceeded, vapor may condense into a water film on the metal surface to corrode the metal. With the increase of the relative humidity, the corrosion proceeds faster and faster. A formula for calculating the relative humidity is: Φ=Z/Zb×100%, where Φ is relative humidity, Z is an absolute moisture content, and Zb is a saturated moisture content. As long as a condensed water film exists in the environmental medium on the surface of the metal material, even if the amount of condensed water is small, an electrochemical corrosion process occurs on the circuit board.
Currently, the relative humidity and dust pollution on the surface of the circuit board inside the communication device may be reduced by two methods, where one method is to apply a conformal coating to the circuit board, and the other method is box type air cooling, so as to enhance environment adaptability of the outdoor installed communication device.
In the former method, the conformal coating is a chemical protective material that resists salt fog, moisture, and dust. The protective film formed by the conformal coating can isolate the circuit board and protect the circuit board against corrosion caused by chemicals, moisture, and other contaminants, improve and extend the service life of the circuit board, and ensure safety and reliability. In practice, the conformal coating is applied to the exterior of the circuit board to be protected, forming a film that is 25 microns to 50 microns thick. The film can protect the circuit board against damage in the case of chemical moisture, salt spray, humidity, high temperature, and the like.
However, an effective protection period of the conformal coating is only 3 to 5 years, which cannot meet the requirement of the communication device product that has a 10-year service life. In addition, the spraying process of the conformal coating is complicated. Tooling is needed to protect connectors on the circuit board and prevent the conformal coating from polluting pins and causing circuit faults; and after completion of spraying, burning-off is required, which increases manufacturing steps. In addition, the protection method is costly. The material of the conformal coating is expensive, and the spraying process also increases the manufacturing cost.
The latter method includes two heat dissipation ways: box type full air blowing or box type full air extraction. The box type full air extraction is shown in FIG. 1. A fan is installed at the air exhaust vent, and air is extracted in the frame body of the communication device, which reduces dust intake in a certain time period and is good for corrosion prevention to some extent.
However, the cooling effect of the box type full air extraction system tends to condense vapor into a water film, which causes circuit board corrosion.